Experimental investigation of n-heptane unsteady-state pyrolysis coking characteristics in microchannel

Coke deposition is considered a challenging problem during hydrocarbon pyrolysis. To get more insights into pyrolysis coking characteristics in the regenerative cooling channels, the unsteady-state experiments of n-heptane were performed in a 2.0 mm (internal diameter) passivated STS304 tube at 873-1073 K and 1.0 MPa with a feeding flow rate of 1.0 mL/min during pyrolysis for 5-60 min. The unsteady-state and spatial coke distributions were obtained. Results showed three distinct stages of coking, namely, catalytic coking, the transition stage, and pyrolytic coking. Coke samples obtained under different temperatures were characterized by element analyzer, scanning electron microscope (SEM), thermogravimeter coupled with a Fourier transform infrared spectroscopy (TG-FTIR), X-ray diffraction (XRD), and Raman spectra apparatus. The coke could be classified into two or three types according to its oxidative activity, and was composed of aromatic carbon and aliphatic hydrocarbons. The morphology of the coke transitions from filamentous to spherical with temperature and pyrolysis time. The C/H ratio of coke increased with the temperature in the catalytic coking stage, but decreased in the pyrolytic coking stage. High H contents and extensive defects of the coke observed at high temperatures could attributed to the generation of large amounts of pyrolytic coke and high coking rate, which resulting a lower degree of dehydrogenation. The presence of metallic substances in the coke verifies the catalytic effect at the beginning of coking and indicates the occurrence of carburization corrosion.

Automated summary

Coke deposition during hydrocarbon pyrolysis was studied through unsteady-state experiments of n-heptane in regenerative cooling channels. The results revealed three distinct stages of coking, with changes in morphology, elemental composition, and oxidative activity of the coke. Metallic substances in the coke indicated catalytic effects and carburization corrosion, while the C/H ratio of coke showed variations with temperature and coking stage.